Project Summary/Abstract Radiation-induced gastrointestinal syndrome (RIGS) occurs when the small intestines are exposed to high doses of radiation. Radiation injury to the intestinal stem cell (ISC) and endothelial compartments impair intestinal regeneration, cause loss of epithelial integrity and mucosal barrier dysfunction. This in turn leads to malabsorption, dehydration, electrolyte imbalances, bacterial translocation, sepsis, and often death. Furthermore, radiation therapy for abdominal tumours is challenging because the small intestine is exquisitely radiosensitive. The intestine?s self-renewal ability and susceptibility to radiation derive from the rapid-cycling ISCs in the crypts. The crypt base columnar (CBC) cells give rise to all the intestinal cell lineages, which are broadly categorized as absorptive or secretory cells. Enteroendocrine cells, which are part of the secretory niche, have been shown to be cryptogenic and injury-inducible. There is also evidence that secretory progenitor cells can revert to CBCs when there is intestinal injury. Our preliminary data indicate that radiation induces the expression of markers associated to the secretory niche. However, the dynamics of the secretory niche plasticity and their relation to CBC cells in the context of radiation injury remain unclear. Moreover, there are no therapies to prevent, mitigate, or treat RIGS or even modest intestinal radiation injury. The EGLN family are cellular oxygen sensors that regulate cell survival and metabolism through the degradation hypoxia-inducible factors (HIFs). HIFs are known to induce tissue remodelling, increase epithelial integrity, stimulate intestinal angiogenesis, and promote stem cell survival, all of which are essential for response to radiation injury. Additionally, HIFs regulate genes required for intestinal barrier function. Our group has shown that stabilization of HIF2, but not HIF1, through inhibition of the EGLN proteins mitigates and protects against RIGS in mice. Yet, the mechanisms by which HIF2 confers this radioprotection remain poorly studied. To gain insight into this mechanism, we performed RNA-seq of HIF2-overexpressing intestinal organoids. We identified Wnt5a, a non-canonical WNT, as a direct transcriptional target of HIF2, but not HIF1. Interestingly, other groups have shown Wnt5a improves colonic crypt regeneration following mechanical injury. We also found that knock-out of Wnt5a decreased the clonogenic potential of ISCs. Thus, we hypothesize that HIF2 induces intestinal regeneration after radiation injury by inducing Wnt5a expression to promote ISC survival. We will examine this hypothesis in two aims. In aim 1 we will determine if HIF2 binds and activates the WNT5A promoter. We will also test if Wnt5a is necessary and sufficient for HIF2-mediated intestinal radioprotection both in vitro and in vivo. In aim 2 we will identify which ISC populations are radioprotected by the HIF2/Wnt5a axis. To do so, we will perform lineage-tracing experiments and single cell transcriptomic analyses. The objective of this proposal is to have sufficient mechanistic understanding of the role the EGLN-HIF2 pathway plays in intestinal regeneration following radiation injury. Insight into this mechanism could aid in new therapies for RIGS, which currently has no therapeutic option.